Cylindrical gear reducers are commonly used equipment in metallurgical and mining machinery. However, the problem of lubricating oil leakage at the shaft end (dynamic seal) has not received sufficient attention from manufacturers, who instead adhere to outdated structural designs, causing significant problems for users. This article discusses some insights and practical experience regarding this issue. 1. Structural Analysis of Dynamic Seals at the Shaft End of Cylindrical Gear Reducers 1.1 Felt Ring Seals: While compact in installation dimensions and requiring low-skill disassembly and assembly, they suffer from rapid wear and require a relatively low shaft linear velocity. Due to their short service life, this type of seal is rarely used for reducer shaft end sealing. 1.2 Gap Throttling Groove Seals: Utilizing the throttling effect of tortuous gaps to generate a seal, these non-contact seals offer long service life and low maintenance requirements, suitable for both high and low speed conditions. However, in practical applications, relying solely on this seal method is not ideal, thus it is only used as an auxiliary sealing method. 1.3 Skeleton Oil Seals: Skeleton oil seals primarily utilize spring rings to tighten the seal. This design ensures that the seal exerts appropriate radial force on the shaft diameter, and that the lip automatically compensates for wear, guaranteeing good sealing performance. The skeleton oil seal is a contact-type seal. Its structure is simple, its installation location is small and compact, and its sealing performance is good. It has a certain adaptability to equipment vibration and shaft diameter eccentricity. However, it has high requirements for shaft diameter, oil seal groove dimensional tolerances, surface roughness, and shaft surface hardness (heat treatment). Furthermore, the viscosity and quality of the lubricating oil, as well as the operating temperature of the equipment, all affect the sealing effect and service life of the oil seal. Additionally, special tools must be used to assemble the oil seal to ensure that the lip does not deform, requiring a high level of craftsmanship. Currently, there are many specifications and sizes of oil seals, but most products are of poor quality and have short lifespans. Even with imported skeleton oil seals, the service life is only about 2 years, often occurring before the major overhaul period of the reducer (generally 3-5 years), when the oil seal has already worn out and lost its sealing function. The cost of simply replacing the skeleton oil seal often exceeds the price of the oil seal itself by several times, even ten times. Another unavoidable factor is that when assembling certain heavy-duty or high-tightness couplings, the hot-fitting method (heating in a furnace or baking with a torch) is commonly used, with temperatures reaching 200-300℃ or even higher. This undoubtedly causes the oil seal lip to deform and melt due to heat after the coupling is installed (due to the small gap), thus losing its sealing function. 1.4 Oil ring seals use a gear shaft as the transmission source, utilizing centrifugal force to throw the outward-flowing oil towards the housing wall, and then return it to the oil sump along the return oil line on the housing wall. Oil ring seals are dynamic seals, suitable for high-speed operation, but their sealing performance weakens at low speeds and is lost when the machine stops. 1.5 Combination seals utilize two or more of the above-mentioned sealing structures, but in practical application, most are not ideal. This is because these sealing structures are not entirely reasonable, or the manufacturing quality of the parts and components, and the installation process are poor. 2. Attempts to improve the sealing structure 2.1 Methods to solve the problem of lubricating oil leakage at the reducer end (dynamic seal) (1) Change the direction of lubricating oil movement; (2) Increase the resistance in the leakage channel; (3) Control the supply of lubricating oil to the bearing; (4) Reduce the pressure difference before and after the sealing point or inside and outside; (5) A combination of two or more of the above. 2.2 Actual requirements (1) Due to the harsh working environment of the reducer in mining machinery: ① Dust enters the reducer and contaminates the lubricating oil, which in turn causes excessive wear between the gears and bearings; ② Dust and lubricating oil are fully mixed under the stirring of the gears, which will affect the lubrication effect and fluidity of the lubricating oil; ③ The accumulation of dust and oil in the return oil channel will block the return flow of the lubricating oil. Therefore, a dustproof structure is necessary. (2) Good lubrication of the gears and bearings of the reducer must be ensured in order to meet the high power and heavy load working nature of mining machinery. At present, most users often use the method of controlling the amount of oil added to avoid lubricating oil leakage. However, due to insufficient lubrication, excessive wear of gears, pitting and bearing overheating are caused. 2.3 Improved Sealing Structure (Taking a reducer with a double outward-extending output shaft as an example) (l) Sealing Structure of Press-in End Cover This structure adopts a combination of spiral seal, oil slinger ring, and skeleton oil seal (in environments with less dust, a gap throttling groove seal structure can be used to form a permanent seal). The sealing principle is as follows: After the reducer is working, splash lubrication is formed, and the supply of lubricating oil to the bearing is controlled by the gap between the oil slinger ring and the housing wall. After the lubricating oil flows through the bearing, part of it flows directly back to the oil sump from the cavity on the lower side of the bearing, and part of it is squeezed into the gap around the inner hole of the end cover under the centrifugal force of the spiral seal ring and the action of the oil flow. However, under the reverse thrust of the spiral, the oil is forced out again and flows back to the oil sump through the oil return groove below the bearing. When the reducer stops, the oil may flow through the spiral in small amounts because the spiral loses its sealing effect, but it will flow back to the oil sump through the oil return groove on the inner side of the end cover, achieving a good sealing effect. The oil seal installed on the outside of the cover plays a role in sealing dust. (2) Sealing structure of embedded end cover The embedded end cover relies on the screw nut to adjust the runout clearance of the bearings at both ends of the reducer. It has the characteristics of quick and convenient maintenance. However, due to its compact structure, it is difficult to add a suitable sealing structure, so it is difficult to solve the oil leakage problem. The improved sealing structure changes it to a fixed clearance adjustment (adding an adjustment shim between the embedded cover and the bearing periphery). As long as the assembly and hoisting process is proper, a suitable axial runout clearance can be guaranteed. Its sealing principle and sealing structure are similar to the sealing structure of the press-in end cover, except that the labyrinth seal is used instead of the spiral seal. 2.4 Factors affecting the sealing effect of the improved sealing structure (1) The width and depth (or diameter) of the oil return groove should be appropriate to facilitate the smooth return of lubricating oil. Without affecting the strength of the reducer housing, the flow cross section of the oil return groove can be appropriately larger; (2) When using a spiral seal, the spiral rotation direction should be adapted to the rotation direction of the reducer gear shaft. When the reducer is used for bidirectional transmission, the spiral seal structure should be avoided and the labyrinth seal structure can be used instead; (3) The sealing effect of the moving seals of each part of the reducer should be improved to ensure the cleanliness of the lubricating oil; (4) The gap value of each sealing channel should be controlled at 0.20-0.30mm. 2.5 Inspection and maintenance precautions (1) When assembling the reducer, apply as little grease as possible to the bearing cavities on both sides of the gear to avoid excess lubricating cells being thrown out after the reducer runs and clogging the oil groove; (2) For dustproof sealing, double-lip oil seals should be used as much as possible, and before the oil seal is installed on the shaft, grease (preferably sodium disulfide) should be applied between the main lip and the dustproof lip to cool down, lubricate the lip, and assist in sealing dust; (3) The amount of oil added to the reducer should be such that the oil surface is submerged to the height of two teeth of the last stage gear. 3. Conclusion Lubricating oil leakage is a crucial indicator of gearbox quality. It poses a significant threat to the safe and reliable operation of the gearbox. If regular maintenance and lubrication are not guaranteed, the loss of lubricating oil will reduce the lifespan of bearings and may even cause damage, leading to excessive gear wear in severe cases. With the increasing emphasis on safe and civilized production in recent years, the issue of gearbox lubricating oil leakage has received considerable attention from technical personnel in factories and mines.